1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, particularly to an electrophotographic photosensitive member having a photosensitive layer which does not cause image deterioration on repeated image formation and exhibits excellent durability.
The present invention relates also to an electrophotographic apparatus, a device unit, and a facsimile machine employing the above electrophotographic photosensitive member.
2. Related Background Art
Many kinds of electrophotographic photosensitive members employing an organic compound as the photoconducting substance have been developed and practically used so far.
Such electrophotographic photosensitive members employing an organic photoconductive substance are expected to be further improved in electrophotographic properties such as sensitivity and photoresponsiveness because of the flexibility in material design thereof, and are advantageous in ease of film formation, high productivity, and relatively low cost.
However, such types of photosensitive members generally have serious disadvantages of low durability. The durability includes electrophotographic durability such as of sensitivity, residual potential, chargeability, blurring of images, etc. and mechanical durability such as resistance to abrasion and scratching of the surface of the photosensitive member by rubbing. It is already known that the lowering of the durability of the electrophotographic properties results mainly from the deterioration of the organic photoconductive substance in the photosensitive layer caused by ozone, NOx, or the like generated by a corona charger.
The deterioration of the organic photoconductive substance leads to crushed images and undecipherable images, namely blurring of images. The deterioration was found to be more remarkable with organic photoconductive substances having a lower oxidation potential since the deterioration results from a kind of oxidation.
From the viewpoint of the electrophotographic properties, polishing of the surface of the photosensitive member to constantly provide a fresh surface is effective for obtaining high-quality images. On the other hand, from the viewpoint of the mechanical durability, less abrasion is desired and therefore a slipping agent is dispersed on the surface of the photosensitive member, which retards the rapid removal of the deteriorated charge-transporting substance and tends to cause blurring of images.
To solve such problems, for example, Japanese Patent Application Laid-Open No. Sho-63-30850 discloses use of a compound having an oxidation potential of not less than 0.6 V as the charge-transporting substance contained in the surface layer of the photosensitive member.
Recently, as the result of prolongation of the life of the photosensitive member, the problem has surfaced that, when the photosensitive member is left standing in a copying machine for a long time after continuous use, the chargeability of a photosensitive member becomes apparently lowered at the portion of the photosensitive member kept in proximity to the corona-discharging charger. This cause a so-called dormant memory phenomenon, giving blank areas in stripes (white bands in positive development, and black bands in reversal development) in the image.
This dormant memory phenomenon is especially remarkable when the employed charge-transporting substance has an oxidation potential of 0.6 V or higher. The occurrence of the aforementioned blurring of images is caused by ozone or NOx generated by corona discharge. In the case where the employed charge-transporting substance has an oxidation potential of less than 0.6 V, the charge-transporting substance itself is oxidized by the ozone or NOx at the surface of the photosensitive member which causes lower resistance, and is liable to cause blurring of images. On the other hand, in the case where the employed charge-transporting substance has an oxidation potential of not less than 0.6 V, the charge-transporting substance is relatively resistant to oxidation, and therefore the ozone or NOx is considered to penetrate deeply into the photosensitive layer to oxidize the charge-generating substance and lower the resistance thereof, whereby hole injection from the support is promoted, resulting in lower apparent potential and causing a dormant memory phenomenon. Although this reduction in the apparent potential is considered to occur during the use of the photosensitive member, the reduction occurs uniformly throughout the surface of the photosensitive member, so that the blanking of images caused by local potential drop is not observed. When the photosensitive member is left standing after continuous operation, local potential drop occurs in the vicinity of the charger where the concentrations of ozone and NOx are extremely high, resulting in blanking of images to a remarkable extent.